LCD having particular dielectric constant relationship between orientation film and LC layer

ABSTRACT

In a liquid crystal display device, the dielectric constants (∈) and resistivities (rho) of the liquid crystal material and the layers of orienting material have such values that the liquid crystal display device can be driven by means of a DC voltage.

BACKGROUND OF THE INVENTION

The invention relates to a liquid crystal display device comprisingpixels and electrodes for driving the pixels, each pixel comprising adisplay element defined by picture electrodes, which display elementcomprises layers of orienting material and a layer of liquid crystalmaterial between the picture electrodes.

Such liquid crystal display devices are generally known and used, forexample, in monitors, but also in portable applications (organizers,mobile telephones).

OBJECTS AND SUMMARY OF THE INVENTION

A known phenomenon in such liquid crystal display devices is thedisplacement of ions in the liquid, so that degradation occurs, whichbecomes manifest as image retention. To prevent this, liquid crystaldisplay devices are driven with an inverting or alternating voltageacross the pixels. This is notably detrimental in portable applicationsbecause the use of an inverting voltage is accompanied by a high energyconsumption and a high battery voltage for the drive electronics. Thisin turn leads to higher costs.

A liquid crystal display device according to the invention ischaracterized in that, for the quotient Q of a dielectric constant∈_(LC) of the liquid crystal material and the dielectric constant of thelayers of orienting material ∈_(ol), it holds that Q=∈_(LC)/∈_(ol)>0.7ρ_(ol)/ρ_(LC), in which, for a liquid crystal material having a negativedielectric anisotropy (Δ∈<0) ∈_(LC), the dielectric constantperpendicular to the directors of the liquid crystal material is(∈_(⊥)), and for a liquid crystal material having a positive dielectricanisotropy (Δ∈>0) ∈_(LC), the dielectric constant parallel to thedirectors of the liquid crystal material is (531 _(∥)) and ρ_(ol) andρ_(LC) are the resistivities of the liquid crystal material and thelayers of orienting material, respectively.

The orientation layer may comprise sub-layers of different material. Inthat case, ρ_(ol) is understood to mean the average resistivity of theorientation layer.

The value Q is preferably between 0.4 and 4, while values of between 1.2and 3 look optimal.

To inhibit image retention even further, a first embodiment ischaracterized in that ρ_(ol)/ρ_(LC)<10 (and preferably<5) at 25° C. Toprevent lateral conduction in the orientation layers, ρ_(ol) is chosento be >10⁷ ohmmeter (T=25° C.).

A further embodiment is characterized in that the liquid crystal displaydevice comprises means for presenting drive voltages in one polarityacross the pixels (DC drive). In this connection, one polarity isunderstood to mean that no measures have been taken to change thepolarity across the pixels during operations over a longer period oftime (e.g. 1000 or 2000 frame times), but measures may be taken to drivethe pixels with opposite polarity when modes are re-used or changed (forexample, switching from or to a standby mode in a display in a portableapplication or switching between use of modes, Internetpages etc. incomputer applications) of the display device.

The inventors have surprisingly found that the display device can beDC-driven without degradation or image retention occurring in saidcombination of dielectric constants of the orienting material and of theliquid crystal material. By approximation, a theoretical explanation canalso be given for this effect. Since the display device is now DCdriven, the electronics for continuously reversing the voltage across apixel may be dispensed with. (Reversing can be restricted to e.g. onceevery minute or five minutes). Moreover, there is no flicker.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows diagrammatically a part of a liquid crystal display device,

FIG. 2 shows an equivalent circuit diagram of the display cell in FIG.1,

FIG. 3 shows the maximum change with respect to time ΔV_(lc,max) acrossthe liquid crystal layer as function of the applied drive voltage, while

FIG. 4 shows the influence of R=ρ_(ol)/ρ_(LC) on the sloped(ΔV_(lc,max))/dV,

FIG. 5 shows the maximum change with respect to time ΔV_(lc,max) as afunction of the applied drive voltage for different liquids, and

FIG. 6 shows diagrammatically a display device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagrammatic cross-section of a part of a liquid crystaldisplay device comprising a liquid crystal cell 1 with a twisted nematicliquid crystal material 2 which is present between two substrates 3, 4of, for example, glass, provided with electrodes 5, 6. The devicefurther comprises two orientation layers 7, 8 which orient the liquidcrystal material on the inner walls of the substrates, in this examplein the direction of the axes of polarization of polarizers (not shown),such that the cell has a twist angle of 90°. In this example, the liquidcrystal material has a positive optical anisotropy and a positivedielectric anisotropy. If the electrodes 5, 6 are energized with anelectric voltage, the molecules, and hence the directors, are directedtowards the field. The display device further comprises a backlight 10(in this example). Alternatively, the display device may be reflective.

In a first approximation, both the combined orientation layers 7, 8 andthe layer of liquid crystal material 2 (LC layer) can be described as aparallel circuit of a resistor and a capacitor, as is shown in FIG. 2.Upon DC drive, the behavior of the voltage across the LC layer(V_(lc)(t), analogous to a step response, will be:

V _(lc)(t)=[R*+((1/C*)−R*).exp.(−t/τ)].V _(lc,cell)  (1)

in which

R*=R _(LC)/(R _(ol) +R _(LC)) and C*=(C _(ol) +C _(LC))/C _(ol)

(R and C represent the resistance per surface unit and the capacitanceper surface unit, respectively, R=ρ.d (ρ: resistivity, d: layerthickness) and C=(∈₀·∈_(r))/d, while τ=R*C*.R_(ol)C_(ol). (V_(lc,cell):voltage applied across the cell).

The maximum change in V_(lc)(t) then is

ΔV _(lc) =V _(lc)(t=0)−V _(lc)(t=∞)=[R*−(1/C*)].V _(lc,cell)  (2)

or

ΔV _(lc,max)=[1/(1+RD)−1/(1+(D/E(V))]  (3),

in which

D=d _(ol) /d _(LC) R=ρ _(ol)/ρ_(LC) and E(V)=∈_(ol)/ρ_(cell)(V)  (4)

The voltage dependence of ∈_(cell)(V) is the result of the change of theaverage dielectric constant in the liquid crystal layer and isapproximately described by:

∈_(cell)(V)=(∈_(∥))−Δ∈.(V _(th) /V)  (5)

(∈_(∥)) is the dielectric constant parallel to the directors (the axesof the liquid crystal molecules) of the liquid crystal material(V=V_(LC,Cell)≈V_(LC))

Δ∈=∈_(∥)−∈_(⊥, V) _(th): threshold voltage of the display cell. Equation(5) applies to V>V_(th).

Since the thickness of the orientation layer is much smaller in practicethan that of the layer of liquid crystalline material (D<<1), the term1/(1+D) in equation (3) can be approximated by (1−D). Equation (3) maythen be written as

ΔV _(lc,max)=[1−RD−(1−(D/E(V))].V=[(1/E(V)−R].DV  (6)

When equation (5) is used for E(V), this yields with (4):

ΔV _(lc,max)=[(∈_(∥)/∈_(ol))−R].DV−(Δ∈/∈_(ol)).DV _(th).  (7)

FIG. 3 shows the variation of ΔV_(lc,max) (formula (6)) for a displaycell with d_(ol)=100 nm, d_(LC)=4 μm, ρ_(ol)=ρ_(LC)=10¹³ ohmmeter,∈_(ol)=3.5, while ∈_(LC)(V) is defined by formula (5), in which ∈_(∥)=8and ∈_(⊥)=3, or ∈_(LC)/∈_(ol)=∈_(∥)/∈_(⊥)=2.3, while ρ_(ol)/ρ_(LC)=1. Itappears from FIG. 3 that the maximum voltage change ΔV_(lc,max) forV>V_(th) varies practically linearly; the slope of the curve has a valuewhich is defined by

d(ΔV _(lc,max))/dV=[(∈_(∥)/∈_(ol))−R.D]  (8)

This derived function is practically constant for V>V_(th), while it hasa value of 0 for ∈_(LC)/∈_(ol)=ρ_(ol)/ρ_(LC). FIG. 4 shows the influenceof R=ρ_(ol)/ρ_(LC) on d(ΔV_(lc,max))/dV. The Figure shows that forR<2.7, the slope of the curve reaches said constant value fairly rapidly(at R˜0,1). Since the maximum change of the drive voltage remainslimited to approximately 4 V (in a voltage range between V_(th) and thesaturation voltage V_(sat)) for (super)twisted nematic LCDs, and themaximum value of the slope is of the order of 0.06, voltage correctionsremain limited to values of approximately 0.25 V (see also FIG. 3). Amaximum change of the slope of 0.10 is admissible; at a small number ofgrey values and an average correction voltage for all grey values, evena larger slope may be admitted.

It is also apparent from FIG. 4 that, for R>2.7, the value of the sloperapidly increases (in absolute value). Up to R≈5 (which corresponds to∈_(LC)/∈_(ol)=0.7 ρ_(ol)/ρ_(LC)), the maximum value of the slope remainsbelow approximately 0.06, so that the same considerations apply asabove.

To prevent image retention, ρ_(LC) should not be too high with respectto ρ_(ol). Therefore, preferably, ρ_(ol)/ρ_(LC)<10 (and <5 in practice).To prevent lateral conduction occurring on a substrate between theelectrodes in a display device having a plurality of electrodes, ρ_(ol)is at least 10⁷ ohmmeter (and preferably at least 10⁸ ohmmeter).

FIG. 5 shows measured values for the voltage ΔV_(lc,max) as a functionof the (DC) drive voltage for different types of display cells(different combinations of materials for the orientation layers and theliquid crystal material). In all cases, ∈_(LC)/∈_(ol)=∈_(∥/∈) _(ol)approximately 8/3≈2.7 and R=ρ_(ol)/ρ_(LC)≈1.5, so that it holds that∈_(LC)/∈_(ol)≈1.8 ρ_(ol)/ρ_(LC). Also for this comparatively high valueof R, it appears that constant values of the slopes d(ΔV_(lc,max))/dVare found. Moreover, it appeared that these slopes did not change at achange of the temperature (measurements at 30 and 60° C.).

EXAMPLES

(In all examples, the LC layer has a thickness of 4 μm and theorientation layers have an overall thickness of 0.1 μm).

1. For a cell with a liquid crystal material with ∈_(∥)=8 and∈_(ol)=3.6, it holds that ∈_(LC)/∈_(ol)=∈_(∥)/∈_(ol)≈2.22. Furthermoreit holds that ρ_(ol)=ρ_(LC)=3.10¹² ohmmeter, so that it holds thatρ_(ol)/ρ_(LC)=1 and ∈_(LC)/∈_(ol)≈2.22 ρ_(ol)/ρ_(LC). For the sloped(ΔV_(lc,max))/dV, a value of 0.025 was found.

2. For a cell with a liquid crystal material, with ∈₈₁ =8 and∈_(ol)=3.6, it holds that ∈_(LC)/∈_(ol)=∈_(∥)/∈_(ol)≈2.22. Furthermoreit holds that ρ_(ol)=4.10¹³ ohmmeter en ρ_(LC)=5.10¹³ ohmmeter, so thatit holds that ρ_(ol)/ρ_(LC)=0.8 and ρ_(LC)/ρ_(ol)≈2.77 ρ_(ol)/ρ_(LC).For the slope d(ΔV_(lc,max))/dV, a value of 0.03 was found.

3. For a cell with a liquid crystal material with ∈_(∥)=8.7 and∈_(ol)=3.6, it holds that ∈_(LC)/∈_(ol)=∈_(∥)/∈_(ol)≈2.42. Furthermoreit holds that ρ_(ol)=4.10¹² ohmmeter and ρ_(LC)=6.10¹² ohmmeter, so thatit holds that ρ_(ol)/ρ_(LC)=0.66 and ∈_(LC)/∈_(ol)26 3.66 ρ_(ol)/ρ_(LC).For the slope d(ΔV_(lc,max))/dV, a value of 0.04 was found.

It appears from the first three examples that d(ΔV_(lc,max))/dVdecreases with a decreasing value of Q. By means of a computersimulation, which also corresponded to the other examples, it was shownthat a slope of approximately zero is reached at Q≈1.5. For 1<Q<1.8, theslope is substantially negligible, certainly when ρ_(ol)≦ρ_(LC).

For a cell with a liquid crystal material with ∈_(∥)=8 and ∈_(ol)=3, itholds that ∈_(LC)/∈_(ol)=∈_(∥)/∈_(ol)26 2.66. Furthermore it holds thatρ_(ol)=1.8.10¹² ohmmeter en ρ_(LC)=1.5.10¹² ohmmeter, so that it holdsthat ρ_(ol)/ρ_(LC)=1.2 and ∈_(LC)/ρ_(ol)≈2.22 ρ_(ol)/ρ_(LC). For theslope d(ΔV_(lc,max))/dV, a value of 0.04 was found.

Counterexample 1: For a cell with a liquid crystal material with∈_(∥)=8.3 and ∈_(ol)=3, it holds that ∈_(LC)/∈_(ol)=∈_(∥)/∈_(ol)≈2.77.Furthermore it holds that ρ_(ol)=1.8.10¹⁵ ohmmeter and ρ_(LC)=10¹⁴ohmmeter, so that it holds that ρ_(ol)/ρ_(LC)=10 and ∈_(LC)/∈_(ol)≈0.27.ρ_(ol)/ρ_(LC). For the slope d(ΔV_(lc,max))/dV, a value of 0.20 wasfound.

Counterexample 2: For a cell with a liquid crystal material with ∈_(∥)=8and ∈_(ol)=3.5, it holds that ∈_(LC)/∈_(ol)=∈_(∥)/∈_(ol)≈2.29.Furthermore it holds that ρ_(ol)=10¹³ ohmmeter and ρ_(LC)=3.10¹²ohmmeter, so that it holds that ρ_(ol)/ρ_(LC)=3.33 and∈_(LC)/∈_(ol)≈0.69. ρ_(ol)/ρ_(LC). For the slope d(ΔV_(lc,max))/dV, avalue of 0.10 was found.

FIG. 6 is an equivalent circuit diagram of a part of a display device 1in which the invention has been applied. This device comprises a matrixof pixels 18 at the area of crossings of m row or selection electrodes 5and n column or data electrodes 6. The row electrodes are successivelyselected by means of a row driver 16, while the column electrodes areprovided with data via a data register 15. If necessary, incoming datasignals 11 are first processed for this purpose in a processor 12.Mutual synchronization between the row driver 16 and the data register15 takes place via drive lines 17.

Drive signals from the row driver 16 select the row electrodes 5. Thesignal present at the column electrode 6 defines the information to bedisplayed. Although a display device of the passive type is shown, theinvention also applies to an active matrix. In that case, separatepixels are connected to the row electrodes via switches (for example,thin-film transistors or two-pole circuits). Selection may also takeplace by means of plasma channels (Plasma Addressed Liquid CrystalDisplay).

As shown above, the values of ΔV_(lc,max) remain within admissiblelimits at the chosen value for the slope d(ΔV_(lc,max))/dV. The value ofΔV_(lc,max) is dependent on the presented voltage (the grey value). In asimple solution, either all voltages on the column electrodes or allvoltages on the row electrodes may be given the same correction voltage(offset). This correction voltage will usually be determined by theaverage value of the voltage range to be used.

In a device in which many grey scales are displayed, the data voltage onthe column electrode is adapted preferably for each value of theincoming signal 11, for example, by means of a look-up table 13 in theprocessor 12.

Although the above-mentioned examples are based on liquid crystalmaterials having a positive dielectric anisotropy, a similarconsideration applies to liquid crystal materials having a negativedielectric anisotropy.

The value ΔV_(lc,max) is also dependent on the voltage across the cell.In color images, a mixed color is formed by the images of, for example,a red, a green and a blue pixel. If, for example, the red pixel receivesa low voltage (maximum transmission in the case of crossed polarizers),while the two other pixels receive, for example, an average voltage, onwhich average voltage also an average correction voltage for all pixelsis based, this leads to discoloration due to a too high correction forthe red pixel. Image retention may also occur when the device is drivenwith the same information for a longer period of time. This is largelyprevented by driving the display device color-sequentially. In thatcase, the backlight 10 supplies one of the three colors in the case of afull color display by means of three sub-images (red, green, blue) for ⅓of a frame time, while the liquid crystal display device comprisesinformation of the relevant sub-image. Since the voltage across thepixel is generally different for the three sub-images, theabove-mentioned image retention occurs less rapidly. This is of coursealso possible with images composed of two sub-images of a differentcolor.

In summary, the invention relates to a liquid crystal display devicewhose dielectric constants ∈_(LC) of the liquid crystal material and ofthe layers of orienting material, as well as the resistivity of theliquid crystal material and the layers of orienting material are chosento be such that the display device can be driven by means of a DCvoltage.

The invention resides in each and every novel characteristic feature andeach and every combination of characteristic features.

What is claimed is:
 1. A liquid crystal display device comprising pixelsand electrodes for driving the pixels, each pixel comprising a displayelement defined by picture electrodes, which display element compriseslayers of orienting material and a layer of liquid crystal materialbetween the picture electrodes, characterized in that, for the quotientQ of a dielectric constant ∈_(LC) of the liquid crystal material and thedielectric constant of the layers of orienting material ∈_(ol) it holdsthat Q=∈_(LC)/∈_(ol)>0.7ρ_(ol)/ρ_(LC), in which, for a liquid crystalmaterial having a negative dielectric anisotropy (Δ∈<0)∈_(LC), thedielectric constant perpendicular to the directors of the liquid crystalmaterial is (∈_(⊥)), and for a liquid crystal material having a positivedielectric anisotropy (Δ∈>0)∈_(LC), the dielectric constant parallel tothe directors of the liquid crystal material is (∈_(∥)) and ρ_(ol) andρ_(LC) are the resistivities of the liquid crystal material and thelayers of orienting material, respectively, and wherein the displaydevice is DC driven.
 2. A liquid crystal display device as claimed inclaim 1, characterized in that Q=∈_(LC)/∈_(ol)≧ρ_(ol)/ρ_(LC).
 3. Aliquid crystal display device as claimed in claim 1, characterized inthat Q=∈_(LC)/∈_(ol) has a value of between 1 and
 4. 4. A liquid crystaldisplay device as claimed in claim 3, characterized in that 1≦Q≦1.8. 5.A liquid crystal display device as claimed in claim 1, characterized inthat ρ_(ol)/ρ_(LC)<10 (T=25° C.).
 6. A liquid crystal display device asclaimed in claim 1, characterized in that ρ_(ol) is at least 10⁷ohmmeter (T=25° C.).
 7. A liquid crystal display device as claimed inclaim 1, characterized in that the display device comprises means forpresenting drive voltages in one polarity across the pixels.
 8. A liquidcrystal display device as claimed in claim 7, characterized in that thedisplay device comprises drive means for presenting row selectionsignals to the row electrodes and drive means for presenting datasignals to the column electrodes.
 9. A liquid crystal display device asclaimed in claim 8, characterized in that the drive means for presentingdata signals comprise correction means for adapting the voltage of thedata signals in dependence upon presented data.
 10. A liquid crystaldisplay device as claimed in claim 7, characterized in that the displaydevice is provided with an illumination unit comprising means forsequentially presenting light in at least two wavelength ranges, and thedrive means are provided with means for presenting, to the displaydevice, the data signals of a wavelength range-associated sub-image ofan image to be displayed.
 11. A liquid crystal display device as claimedin claim 1, characterized in that the display device comprises drivingmeans for inverting the voltages over the display elements at switchingon the display device or after a period of one minute.